The production of hydrogen by the steam reforming of hydrocarbons is well known. In the basic process, a hydrocarbon, or a mixture of hydrocarbons, is initially treated to remove trace contaminants, such as sulfur and olefins, which would adversely affect the reformer catalyst. Methane is a preferred starting material since it has a higher proportion of hydrogen than other hydrocarbons. However, C.sub.3-4 hydrocarbons or others readily available might be utilized as well, although they are primarily converted to methane in the furnace. Since the object is the production of hydrogen, saturated hydrocarbons, i.e. alkanes, are preferred source materials.
The pretreated hydrocarbon is typically compressed, e.g. to about 200 to 400 psig, and combined with high pressure steam, which is at about 700 psig, before entering the reformer furnace. The reformer itself conventionally contains tubes packed with catalyst through which the steam/hydrocarbon mixture passes. An elevated temperature, e.g. about 860.degree. C. is maintained to drive the reaction which is endothermic.
The effluent from the reformer furnace is principally hydrogen, carbon monoxide and carbon dioxide in proportion close to equilibrium amounts at the furnace temperature and pressure with a minor amount of methane. The effluent is conventionally introduced into a one- or two-stage shift reactor to form additional hydrogen and carbon dioxide. The shift reactor converts the carbon monoxide to carbon dioxide with the liberation of additional hydrogen by reaction at high temperature in the presence of steam. The combination of hydrogen steam reformer and shift converter is well known to those of ordinary skill in the art.
There have been proposed a number of schemes for treating the effluent from the shift converter to recover hydrogen and carbon dioxide therefrom. As yet, none of these variations has attained maximum efficiency.
In one such method, the shift converter effluent, which comprises hydrogen, carbon dioxide and water with minor quantities of methane and carbon monoxide is introduced into a conventional absorption unit for carbon dioxide. Such a unit operates on the well-known amine wash or Benfield processes wherein carbon dioxide is removed from the effluent by dissolution in an absorbent solution, i.e. an amine solution or potassium carbonate solution, respectively Conventionally, such units remove about 95 percent of the carbon dioxide in the shift converter effluent.
The effluent from the carbon dioxide absorption unit is introduced into a pressure swing adsorption (PSA) unit. PSA is a well-known process for separating the components of a mixture of gases as a result of the difference in the degree of adsorption among them on a particulate adsorbent retained in a stationary bed. Typically, two or more such beds are operated in a cyclic process comprising adsorption under pressure and desorption under comparatively lower pressure or vacuum. The desired component or components of the gas mixture can be obtained during either of these stages The cycle may contain other steps in addition to the fundamental steps of adsorption and regeneration, and it is commonplace where such a unit contains more than two adsorbent beds to have two beds cycled 180.degree. out of phase, thereby providing a psuedo-continuous flow of desired product.
Conventionally, the effluent from the PSA unit, which comprises carbon monoxide, the hydrocarbon, i.e. methane, hydrogen and carbon dioxide, is returned to the steam reformer and combusted to obtain energy for use therein. There are several disadvantages to this process. First and foremost, the hydrogen which is not removed in the PSA unit, typically about 25 percent, is not recovered, but is lost in the recycle gas which is combusted in the steam reformer. There is unavoidably some loss of hydrogen in the absorber unit due to dissolution thereof in the absorber solution. The combined losses in hydrogen can amount to as much as 40 percent and are generally in the range of 20 to 40 percent. It will be appreciated that these percentages are approximations and can vary depending on the efficiencies of the PSA unit. In addition, the absorber/stripper unit has a significant capital and operating cost which must be factored into the process. There is also carryover of the absorber solution in the effluent to the PSA unit which represents a source of impurity necessitating a pretreatment step for its removal. Finally, the carbon dioxide product of such a process is not of high purity, i.e. not food grade.
In an alternative process disclosed in Sircar, U.S. Pat. No. Re. 31,014, reissued Aug. 17, 1982, the effluent from the shift converter is passed through a complex two-stage PSA unit wherein high purity hydrogen is recovered from the second PSA stage and high purity carbon dioxide is recovered from the first PSA stage during vacuum regeneration. This system is disadvantageous in that, during the production cycle, i.e. the adsorption step, feed flows through both stages concurrently, whereby any ingress of air into the first stage will pass through to the second stage. The oxygen that does not adsorb onto the adsorbent contained therein will pass through and contaminate the hydrogen, a problem if it is desired to prepare merchant grade (99.999 percent purity) hydrogen. Although the product purities of this process are high, they do not meet merchant grade specifications. Operation of the process to produce merchant grade product would significantly decrease product recoveries. Further, this process, like that described above, contemplates return and combustion of the PSA effluent in the steam reformer.
Although the above-described process provides a means of obtaining high purity carbon dioxide and hydrogen from a steam reformer effluent or similar gas mixture, there is considerable room for improvement in quantity and purity of products recovered. Such an improvement is provided in accordance with the present invention.